1. Field of Invention
This invention relates to methods and controls for providing outside air ventilation and vapor compression cooling to commercial and residential buildings, and more specifically, for nighttime pre-cooling of commercial and residential buildings utilizing outside air ventilation and vapor compression cooling.
2. Description of Relevant Art
Utilities and other power providers are striving to reduce electricity use during periods of peak electricity demand because of the high cost of on-peak power and the risk of brown-outs stemming from capacity shortages. Recent studies have predicted an increased rate of power outages in future years based on the inability of power grids to supply increasing power demands. Utility peak capacity shortages are reaching crisis proportions in many areas of the U.S., with utilities resorting to programs for cycling air conditioning equipment off during periods of high peak demand. Methods for reducing power loads are needed to conserve energy and enhance energy independence, as well as to minimize health and safety risks and economic disruptions resulting from power outages. A favorable outcome of the widespread use of ventilation cooling is the reduction of the peak electric load and mitigating the need to construct new power plants.
Summer cooling load is responsible for the greatest share of electricity demand, so methods for reducing this load have economic value and improve electric system reliability. In most summer climates electrical use peaks between about 12 PM and 8 PM. Pre-cooling buildings during the nighttime to a temperature that may be below the normal thermostat setting can be used to shift some or all of the cooling load to off-peak periods. This process involves reducing the temperature of the building mass during morning nighttime hours so that it can absorb heat later in the day thereby maintaining acceptable indoor temperatures with reduced cooling load during the daytime. Conventional methods of nighttime pre-cooling are disclosed in U.S. Pat. No. 5,065,585 issued to Wylie et al. and in U.S. Pat. No. 5,902,183 issued to D'Souza, the subject matter of which are incorporated herein by reference. These systems utilize controllers that measure indoor and outdoor air temperatures and use these measurements to control when the ventilation systems should be operated. Both systems ventilate with outside air when outdoor air temperature is cooler than indoor air temperature by a selected temperature differential, and as long as the indoor air temperature is above a fixed low-limit temperature setting. Conventional thermostats have no provisions for preventing operation specifically during utility peak power use periods or during power shortage alerts. These systems do not provide for optimal reduction of electricity demand during peak afternoon periods. Also, these controls provide no feedback to encourage lower indoor temperature settings that would avert air conditioner use on hot days.
To reduce the electricity demand during periods of peak daytime usage, the pre-cooling process may require the interior space of a building be cooled to a lower temperature than the normal cooling thermostat setting. However, a user will limit this low temperature to maintain an acceptable level of comfort. Pre-cooling can be accomplished by ventilating the space either with cool outdoor air, or with air that is cooled by vapor compression processes. The amount of pre-cooling required to reduce electricity demand during peak afternoon periods depends on current weather conditions.
Optimal pre-cooling requires careful control. Too much pre-cooling uses excessive energy and compromises comfort; too little results in excessive peak period energy use. To achieve the proper balance of pre-cooling utilizing outside air ventilation and pre-cooling utilizing conventional vapor compression cooling requires that the next day's temperature conditions be forecasted in order that the correct amount of pre-cooling can be estimated and subsequently applied. Pre-cooling can be regulated by adjusting the temperature to which the building is pre-cooled, and adjusting the cooling rate for example, by adjusting the volume of air supply during the cooling process. Electronically commutated, variable speed motors (ECM's) allow the airflow during the cooling process to be varied to increase fan efficiency and achieve energy savings during the cooling process.
Conventional methods and control strategies for pre-cooling commercial and residential buildings do not provide for optimal reduction of electricity demand during peak afternoon periods. For instance, conventional methods of setting thermostat schedules are typically performed without considering the use of pre-cooling to reduce electricity consumption during periods of peak usage. Conventional systems that utilize outside air ventilation for pre-cooling typically utilize a simple indoor-outdoor temperature differential control to initiate cooling, and a fixed low limit temperature control to prevent over-cooling. There is a need for improved methods and controls for pre-cooling buildings to minimize the costs of electricity consumption and to reduce electricity demands during periods of peak electricity usage.